Electrochemical measurements and in situ scanning tunneling microscopy (STM) are performed to establish a structure-reactivity correlation for peroxide or dioxygen reduction on underpotentially deposited (upd) Bi, Tl, and Pb on Au(111) in 0.1 M $HClO_4$.
The four-electron electroreduction of dioxygen to water on the Bi upd adlattices has been studied by deliberately poisoning the adlattices during the course of electroreduction activity with thiocyanate and ethanethiol. The diminution in reduction activity was monitored using chronoamperometry. For $SCN^-$ to Bi upd, the drop in current could be modeled using a Langmuir kinetic expression yielding a rate constant for adsorption of $1.1×10^4 s^{-1}M^{-1}$. The rate for ethanethiol could not be measured but must be at least two orders of magnitude faster. STM images of the surface obtained following introduction of $SCN^-$ revealed a (4×4) adlattice, which was partially (6%) defected. The percentage of defects agreed well with the percentage of residual current found at long times (4%) leading us to associate these defects with sites of catalytic activity. STM images obtained from surfaces poisoned with ethanethiol revealed two lattices: a (8×8) structure which was unstable and a more stable ($\surd 57×3$) structure which appeared to exhibit thiols lying flat on the surface. IR studies of the $SCN^-$ poisoned surface showed that the $SCN^-$ was S-bound to the surface at almost the same energy as that expected from $SCN^-$ bound to Au(111). XPS measurements on emersed samples showed that Bi and S were present on the surface. Analysis of these data suggests that the site of dioxygen association with the (2×2)-Bi unpoisoned surface is the lone uncoordinated Au atom in the (2×2) unit cell.
With respect to the Tl upd system, at the potential of catalytic activity toward the $O_2$ reduction, STM reveals the presence of Tl islands on the Au(111) terrace with height of 0.24 ± 0.03 nm. These islands expand in size and number, as the potential becomes more negative, but the full Tl monolayer formed at -0.2 V vs. NHE is catalytically inactive. Ethanethiol (EtSH) significantly inhibits the $H_2O_2$ reduction, and the kinetics and thermodynamics of EtSH adsorption on the Tl upd are quantitatively analyzed. STM shows that EtSH introduction leads to the formation of a 0.15 nm high terrace along the edges of the Tl islands. This terrace is assigned to EtSH bound to the Au surface near the Tl islands with the alkyl chain oriented roughly perpendicular to the surface. These results show that edge sites around the Tl island are the active site of catalytic $O_2$ reduction by Tl upd on Au(111).
Lastly, the Pb upd system shows somewhat similar behavior to the Tl upd system. While STM imaging reveals the presence of Pb islands with height of 0.25 ± 0.05 nm at the potential of highest catalytic activity toward the $O_2$ reduction, the full Tl monolayer formed at -0.03 V vs. NHE shows about half the activity of the Pb island. Ethanethiol (EtSH) significantly but not completely inhibits the $H_2O_2$ reduction activity of the Pb island structure. STM shows that EtSH introduction leads to the formation of a 0.13-nm high terrace along the edges of the Pb islands, which is assigned to EtSH bound to the Au surface near the Pb islands with the alkyl chain oriented roughly perpendicular to the surface. These results show that edge sites around the Pb island are the active site of catalysis, though the sites atop the Pb islands also seem to take part in catalytic $O_2$ reduction by Pb upd on Au(111).
산소환원촉매의 원자구조와 그 활성을 연관시키기 위해 다양한 실험을 수행하였다.
Bi upd 시스템의 경우, $SCN^-$ 와 EtSH가 산소환원반응에 대한 (2×2)-Bi upd 구조의 반응활성을 차단시킨다. 이러한 반응활성의 감소는 주입해준 화합물에 의한 촉매 표면 구조의 변화에 기인한다. SCN- 을 넣어 줄때 bare Au site에 흡착하면서 반응성을 감소시키는 것으로 미루어 (2×2)-Bi upd 구조에서 lone Au site가 산소환원반응에 대한 활성 지점으로 작용한다고 추정된다.
Tl upd 및 Pb upd 시스템의 경우, 산소환원반응에 대해 활성을 보이는 구조는 특유의 island 구조임이 STM을 통해 관찰되었다. EtSH을 주입하면 반응성이 상당히 감소하고, STM 사진 상에서 island 구조의 edge site에 EtSH가 흡착함이 관찰되었다. EtSH의 환원 탈착이 예상되는 전위에서 EtSH의 흡착이 관찰된다는 것은 island 구조에 근접해 있는 Au site가 양전하로 분극되어있음을 의미한다.
Bi, Tl, Pb upd 시스템 모두에서 흡착된 금속과 인접하는 Au site가 이루는 heterobimetallic site가 산소환원반응에서 중요한 역할을 한다. 실제 연료전지의 산소환원전극에 이용되는 촉매물질의 개발에서도 두가지 이상 금속의 합금 형태의 촉매 개발을 통해 촉매의 성능 및 경제성을 향상시킬 수 있으리라 전망한다.
